The present invention relates generally to thrust bearings for pumps and turbines. More specifically, the present invention relates to an improved thrust bearing for accommodating axial thrust acting on the rotors of pumps.
Pumps are rotating machines that increase the pressure of fluids traveling through the pump casing. An impeller or series of impellers are mounted on a shaft within the pump casing. The rotating impellers increase the pressure on the fluid as the fluid travels through the casing. A motor also having a shaft is coupled to the pump shaft to provide the rotating movement.
Barrel pumps are a type of centrifugal pump that use many impeller stages mounted on a common shaft. The impellers face the same direction and are separated by a diffuser, cross over channel and return channel. The shaft with the impellers is mounted into a casing. Low pressure fluid enters the casing at one end of the impeller array and passes through each impeller. The fluid pressure is successively increased by each impeller. At the opposite end of the impeller array the fluid exits the casing from the discharge outlet at a high pressure.
Various forces act on the impeller shaft during operation of the pump. For long shafts with a number of impellers, the shaft must be supported at intermediate points along its length to prevent excessive sagging or curvature. Commonly, bushings that closely fit around the shaft are mounted between the impellers. The bushings act as bearings to counteract the radial forces and to maintain the desired radial position of the shaft.
Axial thrust also acts on the impeller during operation. The axial thrust is additive for each impeller. Since the impellers are attached to the shaft, very strong axial forces may develop along the shaft.
Another force acting on the shaft is an axial force generated by the difference between the low pressure at the inlet of the pump and the high pressure at the outlet of the pump. This axial force, depending on the pump configuration, is typically in the same direction as the axial thrust generated by each impeller. The axial thrust tends to bend the shaft if the thrust bearing is at the inlet end of the shaft.
In pump design, it is desirable to provide a shaft having the smallest diameter possible. By providing a small diameter shaft the amount of flow permitted in the impeller inlet region, which is sometimes called the eye, is maximized. By reducing the shaft diameter, however, the tendency for the shaft to bend is increased.
One way in which axial thrust is accommodated is to allow the bearings of the motor driving the pump to absorb any axial thrust. Because of the high thrust developed by some pumps, special motors may be required to accommodate the axial thrust. By providing a special motor, the typical life of the motor is increased. The cost of the motor is also increased.
Another way in which axial thrust is accommodated in pumps is that a bearing assembly is located between the shaft of the pump and the shaft of the driving motor. Typically, the bearings are rolling contact type such as ball or roller bearings that are lubricated by oil or grease. The bearing housing is independently supported to transmit the thrust force from the pump shaft to the base plate or other structure supporting the bearing housing. One problem associated with a separate bearing assembly is that the components are extremely difficult to manufacture, they require a separate lubrication system and they also require shaft seals. All of these drawbacks undesirably increase the cost of the pump system.
FIG. 1 illustrates a typical barrel pump 10 coupled to a bearing assembly 12. Bearing assembly 12 is coupled to a motor 14. Bearing assembly 12 is used to absorb axial thrust from pump 10 and prevent the axial thrust from reaching motor 14.
Pump 10 has a casing 16. Casing 16 has an inlet port 18 and a discharge port 20. A pump shaft 22 runs longitudinally within casing 16. Radial shaft bearings 25 and 26 support shaft 22 in a rotatable fashion within casing 16. Shaft 22 supports a plurality of impellers 28. Impellers 28, spaced axially apart by shaft sleeves 30, are affixed to shaft 22. Each impeller 28 increases the pressure from the previous stage. Seal 24 seals the end of casing 16 from leaking around shaft 22.
Each impeller stage has an inlet 32 which transmits water toward impeller outlet 34. Impeller inlet 32 is located adjacent to shaft 22. Impeller 28 changes the direction of flow from inlet 32 in an axial direction to a radial direction when fluid is discharged from impeller outlet 34. A diffuser 36 redirects fluid from impeller outlet 34 into a crossover channel 38. Crossover channel 38 transmits fluid into a return channel 40. Return channel 40 redirects water in a radial direction toward shaft 22. Return channel 40 ends at the input to the next impeller stage or at the discharge port 20 in the final stage. An interstage piece 42 separates the various impeller stages.
Each impeller 28 has an impeller hub 44 and impeller ring 48. To reduce leakage between impeller stages, a close clearance is established between each impeller ring 48 and wear ring 46. A close clearance is also established by interstage piece 42 and impeller hub 44 to prevent leakage.
A strut 50 is used to support shaft bearing 26. Strut 50 extends across casing 12 and allows fluid to reach discharge port 20. Strut 50 only minimally effects the flow of fluid exiting pump 10.
Bearing assembly 12 has a shaft 52 and a bearing housing 54. Within bearing housing 54 are a plurality of bearings 56 supporting bearing shaft 52. Seals 58 enclose shaft 52 within housing 54 to prevent leakage of fluid from within bearing housing 54. A bearing housing strut 60 supports bearing housing 54. Bearing housing strut 60 is preferably not coupled to a support for pump 10 or motor 14 so that any forces may be transmitted through bearing housing to strut 60.
Motor 14 has a motor shaft 62. Bearing shaft 52 is coupled to motor shaft 62 and pump shaft 22 by couplings 64.
As is common in prior known barrel pumps, a force is developed in an axial direction as shown by arrow 66. Bearing assembly 12 absorbs axial thrust to prevent the axial thrust from being transmitted to motor 14.